Specifications

Multilayer Switching Overview
Introduction to IPX MLS
13
• Destination mode—The least-specific flow mask mode. The MLS-SE maintains one IPX MLS entry
for each destination IPX address (network and node). All flows to a given destination IPX address
use this IPX MLS entry. Use this mode if no access lists have been configured according to source
IPX address on any of the IPX MLS router interfaces. In this mode the destination IPX address of
the switched flows is displayed, along with the rewrite information: rewritten destination MAC,
rewritten VLAN, and egress port.
• Destination-source mode—The MLS-SE maintains one MLS entry for each destination (network
and node) and source (network only) IPX address pair. All flows between a given source and
destination use this MLS entry regardless of the IPX sockets. Use this mode if an access list exists
on any MLS-RP IPX interfaces that filter on source network.
Note The flow mask mode determines the display of the show mls rp ipx EXEC command. Refer to the
Cisco IOS Switching Services Command Reference for details.
Layer 3-Switched Packet Rewrite
When a packet is Layer 3 switched from a source host to a destination host, the switch (MLS-SE)
performs a packet rewrite based on information it learned from the router (MLS-RP) and then stored in
the MLS cache.
If Host A and Host B are on different VLANs and Host A sends a packet to the MLS-RP to be routed to
Host B, the MLS-SE recognizes that the packet was sent to the MAC address of the MLS-RP. The
MLS-SE then checks the MLS cache and finds the entry matching the flow in question.
When the MLS-SE receives the packet, it is formatted as shown in Table 40:
The MLS-SE rewrites the Layer 2 frame header, changing the destination MAC address to that of Host B
and the source MAC address to that of the MLS-RP (these MAC addresses are stored in the IPX MLS
cache entry for this flow). The Layer 3 IPX addresses remain the same. The MLS-SE rewrites the
switched Layer 3 packets so that they appear to have been routed by a router.
The MLS-SE forwards the rewritten packet to Host B’s VLAN (the destination VLAN is saved in the
IPX MLS cache entry) and Host B receives the packet.
After the MLS-SE performs the packet rewrite, the packet is formatted as shown in Table 41:
Table 40 Layer 3-Switched Packet Header Sent to the MLS-RP
Frame Header Encap IPX Header Payload
Destination Source Length Checksum/
IPX
Length/
Transport
Control
1
1. Transport Control counts the number of times this packet has been routed. If this number is greater than the maximum (the
default is 16), then the packet is dropped.
Packet
Type
Destination
Net/Node/
Socket
Source
Net/Node/
Socket
Data PAD/FCS
MLS-RP
MAC
Host A
MAC
Host B IPX Host A IPX